This invention relates to linkages for drivably engaging a drive shaft and a driven shaft, and more particularly to a linkage for causing controlled, cyclical accelerations and decelerations in the driven shaft, responsive to rotating the drive shaft at a constant speed. A particular embodiment of the invention concerns drivingly engaging the crank shaft and cam shaft of an internal combustion engine to provide accelerations and decelerations in the cam shaft responsive to crank shaft rotation.
It is well known, in connection with conventional four stroke internal combustion engines, that optimal valve timing varies with the speed of the engine. More particularly, the duration over which each of the intake and exhaust valves is open, and the amount of overlap, or time during which both of the valves are open, ideally would vary with engine speed. Typically, however, the cam shaft and crank shaft are associated by a direct drive linkage which fixes cam shaft rotation at onehalf the rate of crank shaft speed. The duration for each valve, constant in terms of angular degrees, decreases in real time as the speed of the engine increases.
This gives rise to the need for a relatively long valve overlap at high engine rpm. In other words, the intake valve is opened before completion of the exhaust stroke and while the exhaust valve is still open, to provide sufficient time for the charge of air and fuel to reach the combustion chamber and enter the chamber as the downwardly traveling piston creates a lower pressure to draw in the charge. Likewise at high rpm, it is desirable and the typical practice for the exhaust valve to be opened before the end of the power stroke, well before bottom dead-center.
Conversely, at low engine rpm, particularly at idling speed, premature opening of the exhaust valve is not needed, and overlap of the intake and exhaust valves is undesirable, since it increases the probability that part of the fuel/air charge will be exhausted, and that some exhaust will be forced backward toward the carburetor, thus to reduce available torque, increase fuel consumption, and increase the undesirable products of combustion which contribute to air pollution. The conventional drive linkage fixes the angular values of duration and overlap, leading to a compromise setting of these values which is optimal neither for idle or high rpm.
Attempts to address this problem are found in the prior art. For example, U.S. Pat. No. 1,622,491 (Coatalen et al) discloses a mechanism for varying the angular relationship of two shafts relative to a third shaft. In particular, epicyclic pinions r and s form part of a drive linkage that can vary the phase of a pump wheel t with respect to a driver 1. FIGS. 3, 4 and 4A show an epicentric linkage of wheels s (two parts) and r, which can be adjusted to vary the angular relation of either wheel p or wheels p and o relative to driver 1. This approach can not affect duration, but does change the phase relationship. In U.S. Pat. No. 1,358,187 (Brewer), a timing mechanism is disclosed in which operation of the exhaust valve in relation to the inlet valve is altered by a gear train which changes the peripheral position of the exhaust valve shaft in relation to the crank shaft.
U.S. Pat. No. 3,888,217 (Hisserich) shows a cam shaft belt drive for variable valve timing, in which slack in a timing belt is taken up by a pair of idler pulleys. The idler pulleys can be displaced to change the angular relation of the drive pulley on the crank shaft relative to a driven pulley on the cam shaft. Separate cam shafts can be provided for the intake and exhaust valve lobes, respectively, in which event the amount of valve overlap can be controlled.
U.S. Pat. No. 3,502,059 (Davis et al) discloses an adjustable gear train in which connector brackets 76 and 78 fix the center-to-center distance between respective pairs of idler gears and idler drive gears, positioned between the crank shafts and a pair of cam shafts. Each bracket is movable to adjust the lash between its associated idler gear and a driven gear mounted on its respective cam shaft.
In an article entitled "Eccentric Cam Drive Varies Valve Timing" (Automotive Engineering, Vol. 88, No. 10, Oct. 1980), a valve timing system is disclosed based on eccentric displacement of a drive sprocket relative to a single overhead cam shaft. The drive sprocket is mounted in a transverse slide which is moved against the force of a tensioning spring, to progressively decrease eccentricity as speed increases. The result is to retard inlet valve openings, advance inlet valve closing, and retard exhaust valve opening. Finally, in an article entitled "Valve Timing With Variable Overlap Control" by Stojek and Stwlorok, separate cam shafts are provided for the inlet and exhaust valves. A helical central gear, driven by the crank shaft through a roller supported chain, cooperates with two helical cam shaft gears to selectively angularly position the cam shaft drive gears with respect to each other. The gears are moved by toothed sleeves, which in turn are driven by microprocessor controlled electric motors, thus enabling a variable valve overlap.
While the above systems in general have demonstrated the possibility of selectively varying valve overlap and, to a limited extent, valve duration, they do not adequately address the need to respond to wide ranging changes in engine rpm.
Therefore, it is an object of the present invention to provide, in an internal combustion engine, a linkage for drivably engaging a cam shaft with a crank shaft in a manner to cyclically accelerate and decelerate the cam shaft responsive to rotating the crank shaft at a constant velocity.
Another object is to provide a means for countering the increase in real time of valve overlap as the rpm of an internal combustion engine decreases.
Another object is to provide optimal intake and exhaust valve timing over a wider range of engine rpm in order to enhance power and fuel economy while reducing engine emissions.
In a broader sense, an object of the invention is to provide a relatively simple means for selectively and cyclically accelerating and decelerating a driven shaft in response to rotating a drive shaft at a constant velocity.